The future satellite missions Sentinel-5 and its precursor will monitor methane column average dry air mole fractions. The ambitious accuracy required on regional scales demands a characterization of the systematic error sources in which spectroscopic uncertainties are crucial. This study investigates how methane and water vapour spectroscopic errors propagate into retrieval errors, showing that spectroscopy-induced errors exceed 0.6% in large parts of the world and are regionally correlated.